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MATE40006 Properties > Dielectrics - Robin > Flashcards

Dielectrics - Robin Flashcards

(60 cards)

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1
Q

what is a dielectric?

A

does not conduct charge carriers , allows the build up of electrostatic charge and retains this charge

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2
Q

what is a paraelectric?

A

any material that becomes polarised in an electric field
when field removed polarisation = 0

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3
Q

electronic polarisation

A

with an electric field, electron cloud is distorted and charge seperation occurs
occurs in every material
dominant when no other mechanisms are present

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4
Q

ionic polarisation

A

with electric field the bond is elastically distoreted and charge seperation increases
only in ionic materials
dominating mechanism in ionic solids

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5
Q

oriental polarisation

A

polar molecules orient themselves within the field
occurs in polar liquids
dominating mechanism in water and other ionic liquids

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6
Q

interfacial polarisation

A

charges migrate to opposite side of the solid
only in ionic solids

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7
Q

the permittivity of a material

A

how easily a material resists the action of a field passing through

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8
Q

capacitors

A

device that stores electrostatic energy
parallel plates separated by vacuum

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9
Q

how do you calculate the capacitance with a dielectric?

A

capacitance plus the charge unit voltage produced by the dielectric

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10
Q

what does a high K0 (static dielectric constant) signify

A

large density
at least one very effective polarisation mechanism
some metal oxides

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11
Q

when does a dielectric breakdown

A

critical field strength, sudden increase in current occurs passing through a dielectric

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12
Q

dielectric strength

A

measure of a materials ability to withstand electric fields
solid (highers)

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13
Q

fill in the blank
the ____ the frequency the ____ difficult it is for a material to respond to polarisation

A

higher more

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14
Q

what happens during strain induced polarisation?

A

dipoles appearing
magnitude of dipole changing as well as its direction

as the lattice parameters change, and the symmetry is lost

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15
Q

heterojunction transistors

A

usually fabricated by using the difference in band gap between two semiconductors

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16
Q

piezoelectric

A

can have spontaneous polarisation from beginning induced polarisation upon application of mechanical stress

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17
Q

pyroelectric

A

has spontaneous polarisation from beginning change in temperature alters spontaneous polarisation

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18
Q

ferroelectric

A

has spontaneous polarisation from beginning can be switched in an electric field

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19
Q

what happens to a piezoelectric if it has centrosymmetry?

A

does not show piezoelectric
means a point group which contains an inversion centre

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20
Q

what is the difference between direct and indirect piezoelectric effect?

A

direct - mechanical stress produces a change in electrical polarisation
indirect - electric field produces a change in mechanical stress

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21
Q

thermal depoling

A

excessive heat with a temperature approaching materials curie temeprature
dipoles start to misalign
above the curie temperature all polarity is lost
avoided by using materials well below their Tc

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22
Q

electrical depoling

A

strong electric field applied in the reverse direction of the polarisation of the material
material depoles

avoided by using field in specific firection and below certain limits

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23
Q

mechanical depoling

A

excessive stress applied to the material
atomic positions are altered to such a degree that all polarisation is lost
avoided by limiting stress

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24
Q

what is the differnce between direct and indirect for pyroelectricity

A

direct can only decrease the effect but indirect can increase/decrease

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25
what happens to ferroelectric as temperature changes?
cubic to tetragonal where there is low symmetry as temperature decreases
26
what is the double well potential?
two equal energy minima exist in the system - a double well with two equal and opposite polarisation directions
27
electrostatic energy
the overall potential energy of a known nymber of static point charges situated in known loactions with respect to each other
28
ferroelectric domain
a region of a ferroelectric material in which all dipoles are aligned in the same direction
29
spin-orbit coupling
the interaction between the spin of each electron and the orbital motion of each electron around the nucleus
30
how do you create a magnetic moment?
total spin and orbital moment > 0
31
magnetic flux density
when a material is placed in a magnetic field, the total flux of magnetic field lines through the material
32
what happens when a diamagnetic material is suspended in a magnetic field gradient?
the energy of the material is increased by the field, therefore material swings away from high field towards low field a small deflection can be observed
33
what is a diamagnetic material and how does it happen?
observed only in materials that have fully paired electrons, spins cancel each other out it is a material which is repelled by a magnetic field
34
diamagnetism as a magnetic material
magnetic moment none susceptibility small and negative
35
when does the meissner effect occur?
when the superconductor is below a critical temperature, the material excludes all magnetic flux from its interior
36
magnetic materials - paramagnetism
magnetic moments - random susceptibility - small and positive
37
what is the difference between the molecular gases of diamagnetics and paramagnetics?
diamagnetic - electrons are paired in the moleculr orbitals, no net magnetic moment paramagnetic - not all electrons are paired in molecular orbitals, net magnetic moment
38
magnetic materials - ferromagnetism
magnetic moments - aligned paralel susceptibility - large
39
what is the curie weiss law?
a system when it has lost its ferromagnetism and become paramagnetic
40
magnetic materials - antiferromagnetism
magnetic moments - aligned (antiparallel) suscepibiltiy - small and positive
41
magnetic materials - ferrimagnetism
magnetic moments - aligned and unequal susceptibility - large
42
magnetic materials - spinglass
magnetic moment - quenched randomness susceptibility - shows a charateristic cusp
43
magnetic materials - helimagnetism
magnetic moments - gradular angular rotation giving a helix susceptibility - shows a maximum at very low temperatures
44
exchange energy
when a ferromagnetic material posses a spontaneous magnetic moment even in the absence of a magnetic field
45
magnetostatic energy meaning
all magnetic moments add up resulting in the largest field
46
domain formation
reduces the magnetostatic energy as the magnetic field is reduced increases the exchange energy as magnetic moments near the domain boundaries cannot align parallel to each other
47
what are the four main contributors to the total magnetic energy?
exchange energy, magnetostatic energy, magnetocrystalline energy magnetostricitve energy
48
magnetocrystalline anisotropy energy
the energy difference between the samples magnetised along easy and hard directions
49
what is magnetostrictive energy?
when a ferromagnetic material is magnetised it undergoes a change in length
50
what is the difference between positive and negative magnetostriction?
postitive - elongates along direction of M negative - contracts along direction of M
51
what is the difference between hard and soft magnets in terms of their hysteresis loop
hard - large coercivity used as permanent magnets increase pinning defect densities soft - small coercivity electromagnets and transformers minimise pinning defect densities
52
what is an application of hard and soft magnets?
hard - permanent magnets soft - frequency ranges AC/DC
53
what are inductors?
coil wound around a core coil induces magnetisation soft magnetic core increases magnetic flux increases inductance by a factor of several thousand
54
what are transformers?
transfers electrical energy between different circuits through em induction
55
ferrites
electrically insulating ferrimagnetic oxides ferrites with low coercivity are used in systems with an oscillating magnetuc field - may induce eddy currents hard ferrites are used to make permanent magnets
56
what is the difference between analog recording and digital recording?
analog tape with embedded magnetic particles magnetised with current proportional to signal magnetisation distribution along tape digital stack of thin films of magnetic material only two stable magnetic states floppy disks hard disk drives
57
HDDS
particulate media small needle like magnetic particles are bonded to metal or polymer thin film media polycrystalline magnetic films deposited on a substrate
58
what are the two magneto - optic phenomena
kerr rotation of polarisation plane of light when it is reflected from a magnetised sample small effect used to observe magnetic domains in bulk sample faraday rotation of the polarisation plane of light when it is transmitted through a magnetised sample larger effect used to observe magnetic domains of thin film samples
59
magnetic semiconductors
semiconducting and magentic behaviour exploit both charge and spin of electrons - spintronics
60
multiferroics
exhibits more than one ferroic property

MATE40006 Properties (5 decks)

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